Black soybean is also referred to as black bean, lu bean, liao bean, or lingwu bean. According to the theory of traditional Chinese medicine, black material can have an effect on kidney, and black soybean, being sweet-warm and innocuous, can affect kidney and spleen as well as heart meridian. Black soybean is thought to have function in tonifying kidney and improving physique, dehumidification and diuresis, as well as resisting aging and prolonging life. Eating black soybean is believed to improve skin, benefits look, replenish marrow, enhance strength, and increase appetite. Researchers have found that black soybean is highly nutritious, and is one of the five kinds of black food which are very popular in the market.
Researchers have shown that there are plentiful nutritious substances in the black soybean hull, and that black soybean has high protein content and low caloric content. Black soybean contains eighteen amino acids, especially the eight amino acids that are necessary for the human body. Black soybean also contains nineteen oleic acids, and the unsaturated fatty acid content of the black soybean is as high as 80%, with an absorptivity of over 95%. Black soybean can lower cholesterol content in blood besides meeting people's demand for fat. Generally, black soybean contains no cholesterol, but rather contains plant sterols, which cannot be absorbed by the human body. Plant sterols restrict the absorption of cholesterol by the body, as well as lower the content of cholesterol in blood. The levels of microelements in black soybean such as zinc, copper, magnesium, molybdenum, selenium, and fluorine also are very high. These microelements play an important role in resisting aging of the human body, lowering blood viscosity, etc. The black soybean hull is black in color and contains anthocyanidin, which is a good source of antioxidants and can eliminate free radicals from the body. Anthocyanidin is thought to have an especially good antioxidation effect in the acid environment of stomach.
Many human diseases are thought to be related to unbalanced free radicals, including cancer, diabetes, and ophthalmologic diseases. In normal situations, the generation and elimination of oxygen-derived free radicals is balanced. When too many oxygen-derived free radicals are generated or the antioxidant system is disabled, however, metabolism of oxygen-derived free radicals in the body will become unbalanced. Polyphenolic compounds are antioxidants, and can be divided into two categories: (1) monomers of polyphenol, including various flavonoids and chlorogenic acid, and composite polyphenolic compounds containing glycoside; and (2) oligomers or polymers polymerized from monomers, generally referred to as tannin. Anthocyanidin is one example of a polyphenol. See, Zheng et al., “Production and Use of Antioxidants,” published by Chemical Industrial Publishing company.
Chinese Patent Application No. 200510036419.3 discloses a method for preparing black soybean seed hull anthocyanin comprising the following steps: 1) peeling black soybean seed hull as the raw material; 2) grinding black soybean seed hull into fine powder with a super-micro pulverizer, and processing the fine powder with a micro-wave machine; 3) extracting the crushed black soybean seed hull anthocyanin with 60-80% ethanol as the solvent; 4) condensing the extracted solution of the black soybean seed hull anthocyanin obtained in step 3 in a vacuum until the concentration of solid substances in the solution is over 40%; 5) degreasing, absorbing, and desorbing the extracted condensed solution obtained in step 4 with 65-75% ethanol, thus obtaining the purified extraction solution of the black soybean seed hull anthocyanin; and 6) condensing the extraction solution obtained in step 5 in a vacuum again until the concentration of solid substances is over 60%, and then freeze-drying the resulting substance to obtain the black soybean seed hull anthocyanin. The yield is said to increase remarkably when the fineness of the black soybean seed hull is over 320 meshes.
Chinese Patent Application No. 88104657.4 discloses methods for extracting a natural brown pigment, including an extracting method using water and an extracting method using ethanol. The extracting method using water is as follows: the seed hull of black soybean or brown soybean is added with 5-15 times of water, and is soaked for 10-24 hours, then is refluxed for 1 hour or boiled for 1-2 hours and then filtered. The aforesaid steps are repeated three times, and the filtrates are combined, decompressed and condensed, then 95% of ethanol is added into the condensed solution, and deposits are filtered after the condensed solution is placed still for a time. The resulting solution is then decompressed, concentrated, and dried to obtain the natural brown pigment. In the extracting method using ethanol, 15%-50% ethanol is used to soak the seed hull, and the other steps are the same as those in the extracting method using water.
Japanese Patent Application No. 2004-238303 discloses a method for extracting a black soybean seed hull extract composition. The method includes the following steps: the black soybean seed hull is extracted in an acid solution with a pH 1-2, then is neutralized until the pH reaches 3.0-4.0, and filtered, then desalted, condensed, and refined. The contents of substances are adjusted through an adding procedure, and then the resulting substance is dried to be the final product. According to the method, the content of the anthocyanin as the effective component is over 10% by weight, and the content of the polyphenol is over 50% by weight determined by UV absorbance analysis.
Although the extracted target substances are not the same for the above extracting methods, the methods have the same general defects. The yield obtained by the extracting method using ethanol is relatively high, but the production cost is also high; the extracting method using water is of low cost, but the yield is also low. Further, the purity of products obtained by extracting methods using water or ethanol is not high. Therefore, an extracting method by which higher yield and products of higher purity can be achieved is desired.